In the metal industry, one of the most challenging problems is the avoidance of ferrite formation while producing high grade calcine from iron-bearing base zinc sulfide concentrates. In the present state of the art, the production of a high grade metal calcine without concomitant formation of ferrites requires very demanding roast conditions and the employment of strong reducing gases, such as CO,H.sub.2, etc., which by their nature also can reduce metal oxides to metals. These ferrite elimination methods, while technically satisfactory when properly controlled, are inherently very cost intensive. To our knowledge, no-one is employing such methods commercially today.
The necessity for minimizing the formation of ferrite resides in the fact that ferrites, which are co-produced during normal high temperature roasting processes by combination of ferric oxide with other metal oxides, are insoluble in the relatively dilute mineral acids commonly employed for leaching metal values from the calcine. In the case of zinc processing, this lack of solubility of ferrites means that the solids residue remaining after the leaching step contains a significant concentration of zinc which, if no steps were taken to recover it, would represent a loss to the process and significantly affect the overall efficiency of the leaching process
Steps that can be taken to liberate the non-ferrous metals in the ferrite usually involve leaching with stronger mineral acid under very intensified leach conditions. The problem with such a leach is that the associated iron values in the ferrite are also co-dissolved and must subsequently be removed and disposed of prior to further beneficiation. For example, in the production of electrolytic zinc, maximum solubility of zinc content of the calcine in the leach liquor is essential, as this has a profound effect on the efficiency and economics of the process. In practice, this implies that the formation of zinc ferrite needs to be minimized during roasting to avoid necessary retreatment of leach residues to recover the zinc This proves difficult to accomplish in practice and does not, therefore, yield the desired results as the formation of zinc ferrite is unavoidable due to its formation being favoured thermodynamically under normal roasting conditions.
While the disadvantages of ferrite formation during roasting may have been altered now with the advent of Jerosite, Goethite and Hematite selective precipitation processes which made it possible and commercially feasible to deal with soluble iron, these iron handling techniques are quite costly as is the intensified leach step which solubilizes the ferrites. It should be recognized that the higher the iron level is in the concentrate, the higher will be the zinc ferrite fraction formed in the calcine, the lower will be the overall extraction of metal during normal leaching, and the more important and costly these iron handling steps will become. It is therefore, clear that a process which would reduce ferritization of base metals, particularly zinc, would represent a marked improvement in the art.
A search has been conducted with respect to the present invention. The following U.S. Pat. Nos. have been located as the closest prior art: 692,148; 872,822; 875,332; 1,401,733; 1,463,901; 1,874,370; 1,930,370; 1,940,912; 3,095,363; 3,181,944; 3,346,364; 3,758,293; 4,201,748; 4,231,791; 4,238,222 and 4,437,884. The applicants do not believe that any one of these references discloses or suggests the present invention.
Additional prior art also is known to the applicants from the prosecution of the aforesaid precursor application Ser. No. 829,927 as follows:
German AS No. 1,199,506 describes a method of recovering zinc from zinc-containing sulfidic ores by roasting the sulfidic ore, solubilizing zinc values and electrolytically producing metallic zinc from the resulting zinc-salt solution. The reference mentions the problem of zinc ferrite formation at high temperatures and long residence times and a desire to avoid the formation of zinc ferrite.
The reference further describes, as prior art, a wet procedure in which zinc-containing sulfidic ore is treated with sulfuric acid and free oxygen under pressure to solubilize the zinc as zinc sulphate while the sulfide sulfur is oxidized to elementary sulfur.
The procedure adopted in the German reference is to effect an initial roasting operation at a temperature which, at a very maximum is 850.degree. C. and preferably is between 750.degree. and 800.degree. C. At those temperatures, it is indicated that only 60 to 90% of the initial sulfide sulfur can be oxidized to SO.sub.2 and then only at relatively slow rates of reaction The residue then is subjected to the oxidative pressure leaching with sulphuric acid in the presence of free oxygen at a temperature above the melting point of sulfur to solubilize the zinc values as zinc sulfate and to convert the residual sulfide sulfur to elemental sulfur. Zinc then is recovered from the leachate.
The use of the temperatures below 850.degree. C. is said to ensure the absence of zinc ferrite formation during the roasting operation. In the present invention, the process is carried out under conventional zinc sulfide roaster conditions of at least about 900.degree. C. and yet zinc ferrite formation is suppressed.
U.S. Pat. No. 3,181,944 also mentions the ferrite formation problem discussed above. The reference purports to describe the roasting of zinc sulfide ores to produce a calcine lower in residual zinc sulfide, zinc sulfate and ferrites, to increase the solubility of the zinc content of the calcine and increase the yield of recoverable zinc. This result is achieved by using a specific fluidized bed procedure at a temperature of approximately 800.degree. to 900.degree. C. in which a substantially complete roast is effected and significant ferrite formation occurs. By contrast, the present invention avoids such ferrite formation without the elaborate processing described in this prior art.